Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U. S. Pat. Nos. 5,585,069, 5,593,838,and 5,603,351,all assigned to the David Sarnoff Research Center, Inc., and hereby incorporated by reference. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels provide the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and them pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis. When used as a printing device, the chemical reagent solutions are replaced by dispersions of cyan, magenta, and yellow pigment, and the array of reaction cells may be considered a viewable display of picture elements, or pixels, comprising mixtures of pigments having the hue of the pixel in the original scene. When contacted with receiver, the capillary force of the receiver fibers pulls the dye from the cells and holds it in the receiver, thus producing a receiver print, or photograph, of the original scene. One problem with this kind of printer is the rendering of an accurate tone scale. The problem comes about because the capillary force of the receiver fibers remove all the pigment solution from the cell, draining it empty. If, for example, a yellow pixel is being printed, the density of the image will be fully yellow. However, in some scenes, a light, or pale yellow is the original scene color. One way to solve this problem might be to stock and pump a number of yellow pigments ranging from very light to dark yellow. Another way to solve the tone scale problem is to print a very small dot of dark yellow and leave white receiver surrounding the dot. The human eye will integrate the white and the small dot of dark yellow leading to an impression of light yellow, provided the dot is small enough. This is the principle upon which the art of color halftone lithographic printing rests. It is sometimes referred to as area modulation of tone scale. However, in order to provide a full tone scale of colors, a high resolution printer is required, with many more dots per inch than would be required if the colors could be printed at different densities. Another solution to the tone scale problem has been provided in the area of ink jet printers, as described in U.S. Pat. No. 5,606,351, by Gilbert A. Hawkins, hereby incorporated by reference. In an ink jet printer, the drop size is determined primarily by the surface tension of the ink and the size of the orifice from which the drop is ejected. The ink jet printer thus has a similar problem with rendition of tone scale. U.S. Pat. No. 5,606,351 overcomes the problem by premixing the colored ink with a colorless ink in the correct proportions to produce a drop of ink of the correct intensity to render tone scale. However, ink jet printers require a relatively high level of power to function, and they tend to be slow since only a few pixels are printed at a time (serial printing), in comparison to the microfluidic printer in which all the pixels are printed simultaneously (parallel printing). Another possible solution to the tone scale problem is described in the above cross referenced copending application, wherein a colorless ink is mixed with the colored inks to make the light colored pixels that are needed for a continuous tone image. The problem with this method is that the receiver receiver to which the inks are being transferred must be removed at just the right time or too much ink may be pulled from the microchannels which convey it to the receiver. This is particularly difficult to time in conditions where the temperature may vary, because the rate of flow of the ink will be temperature sensitive. If too much ink is pulled from the microchannel apparatus, the densities of the image will be too high, and resolution will be lost, because the expanding pixel of ink will blend with the adjacent pixels. This blending may also cause the color saturation to be reduced. Excess ink will not produce a pretty picture.
It would be desirable to have a receiver for rapidly printing a high quality continuous tone image using a microfluidic printing apparatus.